• Hydrogen Reformation
• Hydrogen Storage
• Endnotes

Large scale reformation
Large-scale methods for reforming hydrogen are most compatible with a hydrogen fuel cell infrastructure that demands the production of pure hydrogen to be stored in facilities such as fueling stations. These fueling stations can store hydrogen as a compressed gas or liquid, in addition to converting it from a liquid to a compressed gas at the fueling station.¹ Hydrogen derived from large-scale reformers can also be “injected” into solid hydrogen storage containers such as metal hydrides and nanotubes. The most common method of large-scale reformation relies upon steam-reforming or electrolytic processes. Ninety-five percent of all hydrogen is produced through steam reforming of natural gas.² Hydrogen production processes are most commonly used for industrial applications (primarily as a chemical for petrochemical, electronics, and food industries).³ The Department of Energy explains that most of the hydrogen produced today is consumed on site, such as at an oil refinery. Cost estimates of hydrogen are: $0.32/lb if it is consumed on site, $1.00-1.40/lb for delivered liquid hydrogen, and $1.00–$2.00/lb for hydrogen produced by electrolysis.⁴

Electrolysis technologies refer to the process of splitting water by running an electric current through it to produce hydrogen. Electrolysis can have up to 80-85 percent efficiency.⁵ The cost of electrolysis depends on the cost of electricity being run through the water. Some fuel cells use solar panels to generate electricity (expensive due to the high cost of solar-electric technology) to produce hydrogen, which is used or stored in hydrides or containers.⁶

Steam electrolysis, another process for producing hydrogen, requires energy in the form of heat (2,500 °C) instead of electricity to split water. However, it is often difficult to prevent the water and the hydrogen from recombining. Hydrogen can also be derived from photobiological methods using the natural photosynthetic activity of bacteria and green algae. This process is sometimes called biophotolysis. The University of Hawaii has built a processing plant with a development scale bioreactor.

On-board reformation
Designers of fuel-processing units are faced with two challenges: reforming the hydrogen and cleaning carbon monoxide off the catalyst. These processes need to operate in conjunction so that the expensive catalysts are not deteriorated by the carbon monoxide (a by-product of the reformation process). Much of the research into reforming technologies centers on optimizing the proportions of the fuel, water, air that allow the system to run most efficiently.

“On-board” reformation involves a miniaturized steam-reformation process in which the fuel is heated up and then subjected to a catalyst to liberate the hydrogen. Three main fuel-processing technologies include catalytic steam reformer (vaporization and catalyst, heat external to system), autothermal reformers (vaporization and catalyst, heat derived from fuel), and catalytic partial oxidation reformers (similar to autothermal reformers, although smaller).⁷

Los Alamos National Laboratory has developed solutions to allow proton exchange membrane fuel cells to operate on impure hydrogen fuel. By bleeding low levels of air into the fuel stream, fuel cells were able to run on contaminated hydrogen.⁸ Limiting the amount of carbon monoxide (CO) build-up on the expensive metal catalyst requires gas clean-up units. Estimates range from less than 10 ppm CO as an acceptable level for a proton exchange membrane fuel cell stack⁹ to 50 ppm as a poisonous level for the metal catalyst.¹⁰

Fuels
Methanol is an attractive fuel option because of its high energy density and because it can be stored as a liquid using existing fuel infrastructure. Methanol processors received attention from Argonne National Laboratory, until the program was recently transferred to a partnership between DOE and General Motors Corp. According to the American Methanol Institute, pure methanol (M-100) is much harder to ignite than gasoline and burns at a 60 percent slower rate.¹¹ Methanol proponents cite this fact to show a safety advantage over gasoline.

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Hydrogen Storage

Chemically Stored Hydrogen
Research into chemically stored hydrogen is increasingly popular due to the constraints that fuel processors face. Products such as nanotubes or hydrides are leading the areas of research and are becoming commercially developed as companies discover how to mass produce these storage technologies. The main obstacle in chemically storing hydrogen is the hydrogen to weight ratio of the storage media. That is, it is desirable to store a large amount of hydrogen in the lightest unit. Some estimates place the necessary target for economic on-board hydrogen storage for vehicles at five percent by weight with reversible hydrogen storage at 120°C.¹² In addition to hydrides and nanotubes, other chemical means for storing hydrogen are being explored, such as immersing the hydrogen in a solution of sodium borohydride or sodium tetrahydridoborate.

Hydrides
Hydrides are compounds of hydrogen bound with metals that allow for solid hydrogen storage. According to the Department of Energy, research is currently being conducted on magnesium hydrides. Certain metal alloys such as magnesium nickel, magnesium copper, and iron titanium compounds, absorb hydrogen and release it when heated.¹³ The International Energy Agency Hydrogen Program is making efforts to reduce this temperature required to release the hydrogen to below 80 °C.¹⁴

Nanotube Storage
Nanotubes consist of carbon atoms arranged in a complicated structure that allow for hydrogen storage. The process for adsorbing hydrogen to the carbon containing material in the nanotube is among the variables being tested. Some methods to produce nanotubes include: 1) laser vaporization of carbon that has been treated with a metal such as nickel, cobalt or iron; 2) catalytic chemical vapor deposition; and, 3) electric arc vaporization of a metal-impregnated carbon electrode. As with hydrides the crucial factor for nanotubes is the weight percentage hydrogen of carbon density, (also referred to as the ratio of stored hydrogen to carbon or percentage of its own weight in hydrogen). Estimates of percentage weight of stored hydrogen for nanotubes range from 4%¹⁵ to a claimed 65%. The high estimate is based on a material synthesized by researchers at Northeastern University.¹⁶ Given the broad range of numbers, it appears that techniques for nanotube production are still in a developmental phase.

Liquid, Gas, and Slush Storage of Hydrogen
Traditional means of storing hydrogen include liquid and gas storage. Refrigerating (liquefying) hydrogen to –253 degrees Celsius “uses the equivalent of 25% to 30% of its energy content, and requires special materials and handling. To cool one pound (0.45 kg) of hydrogen requires 5 kWh of electrical energy.”¹⁷ This method of cryogenic storage of hydrogen allows regular commercial shipment by truck and rail. The National Renewable Energy Laboratory (NREL) states that many commercial processes such as glass manufacturing, brazing, heat treating, food hydrogenation, and semiconductor manufacturing are served by deliveries of liquid hydrogen. Liquid hydrogen has also facilitated the U.S. space exploration program.¹⁸

Gaseous hydrogen storage requires less energy than liquid hydrogen storage, but requires high pressures to store adequate amounts. According to NREL, for large-scale use, pressurized hydrogen gas could be stored in caverns, gas fields, and mines. The hydrogen gas could then be piped into individual homes in the same way as natural gas. Though this means of storage is feasible for heating, it is not practical for transportation because the pressurized metal tanks used for storing hydrogen gas for transportation are very expensive.¹⁹

A slush method for storing hydrogen is still in the early stages of development. It is achieved by using a vacuum to evaporate liquid hydrogen, with the temperature falling below the freezing point of –259 degrees Celsius. This creates more dense storage possibilities for hydrogen.²⁰

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